Epidemiological evidence has suggested the hypothesis that the incidence of colon cancer is related to diet and to the metabolic activity of the intestinal microflora. Experimental investigation of this hypothesis requires large numbers of animals maintained for extensive time under different dietary conditions to detect what might be only a small (but significant) difference in tumor incidence. Recently it has been found that histidine auxotroph TA1538 developed by Ames can be maintained at concentrations of 10 to the 8th power per gram in the colon and cecum and appears in this concentration in the feces of otherwise germ-free rats. When these gnotobiotic rats are fed a variety of carcinogens the reversion to histidine independence which indicates the mutagenic response in the Ames mutants can be detected in the lower bowel and the feces. Although the various frameshift and base pair mutants developed by Ames respond to 90 percent of known chemical carcinogens in appropriate pour plate assays, they fail to respond to the colon carcinogens azoxymethane and 1,2-dimethylhydrazine in these assays. This research seeks to determine whether Ames tester strains other than TA1538, e.g., TA98, TA1535, TA1537, TA100 and G-46, will associate in the gastrointestinal tract and will be mutated in situ by these colon carcinogens. Such a system could be a quick, site-specific method of detecting the reactive intermediates in the colon which may initiate chemical carcinogenesis. Since experiments with TA1538 indicate that other constituents of the flora can be added along with the Ames mutants, the system can be used to study the effect of the flora on the mutant response within the intestine. If a mutant response is noted, the site of activation of these colon carcinogens (which are not activated by the liver microsomal system in the pour plate assay) will be sought using homogenates of sections of the intestine. It is hoped that this research will aid the development of a site specific host-mediated assay for the detection of ultimate carcinogens within the colon and for elucidating in a preliminary way the factors governing their formation and disappearance.